1. Field of the Invention
The present invention relates to a lens barrel in which a lens holder advances and recedes in a direction of an optical axis with rotation of a cam ring about the optical axis, and an optical device incorporating the lens barrel.
2. Description of the Related Art
Conventionally, there have been known lens barrels for cameras that carry out zooming and focusing operations by performing lens movement in a direction of an optical axis by driving a cam. Further, another type of lens barrel has been proposed which is provided with a first cam-follower engageable with a first cam groove formed in a cam ring and a second cam-follower engageable with and disengageable from a second cam groove formed in the cam ring, to thereby shorten the length of the cam ring in the optical axis direction without shortening the maximum possible moving distance of the lens barrel (for example, refer to Japanese Laid-Open Patent Publication (Kokai) No. 2004-085932). The lens barrel of this kind is arranged such that corresponding ones of the cam-followers and the cam grooves have some degree of allowance (play) during engagement, so that the lens movement is carried out smoothly in particular when the second cam-follower is brought into or out of engagement with the second cam groove. Due to this allowance, there is a possibility that the optical axis of the lens barrel provided with the cam-followers and that of the cam ring formed with cam grooves may become misaligned, that is, an offset may occur.
Now, a construction as shown in FIG. 10 is referred to by way of example, in which a second-group lens barrel 37 advances and recedes in an optical axis direction according to a pivotal movement of a moving cam ring 3. In this construction, first and second cam grooves 3d and 3e are formed at different positions in the moving cam ring 3 as viewed in the optical direction and have the same basic groove shape, so that the cam grooves extend in parallel to each other and describe the same locus. However, the second cam groove 3e is comprised of an actual cam groove portion extending to a rear end of the moving cam ring 3, and a virtual cam groove portion which is a virtual extension of the actual cam groove portion. That is, the virtual cam groove portion of the cam groove 3e is hypothetically provided at a position outside the moving cam ring 3 where any cam groove cannot actually be formed. Two fixed followers 37a and 37b having tapered front end portions are provided on the outer peripheral portion of the second-group lens barrel 37. When the second-group lens barrel 37 moves in the optical axis direction and reaches a predetermined position, the fixed follower 37b of the second-group lens barrel 37 disengages from the actual cam groove portion of the cam groove 3e and advances to a virtual cam groove portion thereof, or reversely, advances to the actual cam groove portion from the virtual cam groove portion. The other fixed follower portion 37a remains engaged with the first cam groove 3d irrespective of the moving position of the second-group lens barrel 37.
However, since there is allowance between each of the fixed followers 37a and 37b and a corresponding one of the cam grooves 3d and 3e, the second-group lens barrel 37 may offset from the moving cam ring 3, and thus, there is a possibility that a predetermined optical performance cannot be maintained.
With a lens barrel having the above mechanism, in order to not cause negative effects on optical performance, it is necessary to provide an optical design such that the sensitivity to the degree of parallelism between the lens groups becomes small. However, to realize such an optical design, the number of lenses in the construction increases which causes the total length of the lens barrel to become longer and the size of the lens barrel to become larger. This is disadvantageous in terms of the recent demands for low cost, smaller size and lighter weight.
Therefore, in order to shorten the total length of the lens barrel, a solution may be to reduce the number of lenses. This requires an optical design that realizes the lens groups to be highly coaxial with each other, which makes it necessary to design a lens barrel that reduces the allowance between the cam groove and the cam-follower during engagement. However, if the allowance between the cam groove and the cam-follower during engagement is reduced, there is a possibility that the cam-follower will hook on to the cam groove when it disengages from the cam groove or remounts onto the cam groove. In such a case, the base board of the camera body will vibrate microscopically, and such vibration will transfer to the image forming plane. Due to this, when using an electronic viewfinder in an electronic still camera, the photographic screen will shake and cause a so-called image shaking. In a worst-case scenario, the cam-follower will remain hooked to the cam groove, and consequently, the lens barrel may not operate. Reducing the allowance between the cam groove and the cam-follower during engagement imposes restrictions on obtaining high optical performance.